1. Field of the Invention
The present invention relates to a device and method for perception-optimized transmission of audio and video signals.
2. Related Art
In multimedia applications playing-back moving pictures on the computer monitor is becoming ever more important.
The main problem in representing video pictures on today's PC's lies in the bus bandwidth in relation to the large amount of data required for picture formation. Thus a single television picture (640.times.400 pixels) in 24 bit color requires more than 1 megabyte of memory. According to the European television standard, about 23 MB per second are required. From this it is clear that the buses and the hard disks of today's PC's are not suited to this quantity of data. Transfers of video data through ISDN, with a transfer rate of 64 kbits per second set narrow limits. Picture resolution and picture speeds must be reduced. The same applies to the transfer of video data via telephone lines.
In respect of the high data transfer rates, three basic methods have been established to bring video pictures to a PC platform.
The first method solves the problem by displaying a video picture directly on a screen window without transferring it through the PC bus and memory.
The second method is based on extending the bandwidth of the bus by using supercomputers.
The third method solves the problem of high data transfer rates by data compression. Data compression seeks to reduce the data transfer rates sufficiently to enable the existing bandwidths of the buses to be fully utilized, without significant loss in quality. All data compression methods for picture representation are based on the knowledge that the brain can process less picture information than the human eye can accept. As a result, data compression can be carried out to such an extent that the reduction in picture quality cannot be detected by the brain. In this case, less relevant information is simply neglected in data transfer.
There are two types of data compression, namely data compression involving no loss and that where a loss is involved.
Video and audio signals contain redundant information which can be combined and compressed without loss of picture quality. The data compression factor lies between 1.5:1 and 3:1. In contrast to this, in data compression involving loss, signal information which is not important for recognition is simply not transferred. The advantage in this case is a higher compression factor. The disadvantage resides in determining what is important or less important for recognition. What is important or less important is decided by the coder/decoder. The coder can carry out data compression either within individual pictures (intraframe compression) or between several pictures (interframe compression) or by a combination of both methods.
In data compression within individual pictures, a number of techniques exist to determine the data compression factor and the quality of the compression by the way in which they are combined or arranged. Thus the compression techniques include, inter alia, filtering, color-separation conversion, scaling, transforming and run-length coding.
In filtering, the picture is not directly compressed but instead is prepared for data compression. This is generally effected by simply eliminating the high frequencies.
Color-separation conversion, is based on the knowledge that the human eye finds it more difficult to distinguish colors than shades of brightness. Consequently, the picture is subdivided into one luminance component and two chrominance components. The chrominance components are then evaluated at a lower resolution.
In scaling, the three parameters, resolution, color depth and frame rate are changed.
Transformation is based on the co-called DCTS; cosine transformation. In this method, the picture information is sub-divided into 8.times.8 pixel blocks and high and low frequencies.
In run-length encoding, the data are coded without loss of information. Identical bits are replaced by numbers (e.g. 1111222 by 4132).
Data compression through frame sequences is based on the fact that, with running video pictures, the greatest data redundancy is not in individual pictures but between several pictures in a sequence. Many pixels often change only slightly from frame to frame and generally not at all, so that only the difference between the individual frames needs to be coded and not the individual pictures themselves.
MPEG has become internationally widespread as the standard for data compression and decompression of audio and video pictures, particularly for pictures stored on CD. MPEG is based on a combination of the above-described compression processes (intraframe compression and interframe compression). With MPEG, data rates of 1,856,000 bits per second can be processed.
In order to carry out the cosine transformation, MPEG requires a high level of computing power. A further disadvantage of the MPEG process resides in the fact that the maximal achievable compression is limited because of the possible local and time compression at a predetermined resolution and frame rate. In addition, MPEG requires the picture to be completely reconstructed at regular intervals.